SHERMAN, Texas (CNN) -- The bus that overturned Friday morning in northern Texas, killing 15 people, was equipped with a recapped tire on the right front in violation of safety rules, a federal investigator said Friday.

The damaged bus is hauled from the crash scene on a flatbed truck Friday in Sherman, Texas.

"The right tire on the steer axle was a Goodyear tire, and it had been recapped," National Transportation Safety Board Member Debbie Hersman said. Though the trucking industry commonly uses recapped tires to extend the tires' lifespan, "they cannot be placed on the steer axle of the bus," she said.

The tire was the only one of 10 bus tires that was recapped, and it was the only one whose tread separated, she said.

The ban on recapped tires from the steer axle is intended to limit the chance that a sudden loss of tire pressure could cause the driver to lose control of the vehicle, Hersman said.

Asked what caused the Goodyear tire to lose its air, she said, "we don't know the answer to that question; that's why we're here."

Investigators found a blown tire on the bridge and are looking into the possibility that the tire may have caused or contributed to the wreck, a police spokesman said. The right front tire of the extensively damaged bus appeared shredded.

The bus itself was made in 2002 by Motor Coach Industries.

Police estimated that, in addition to the deaths, 33 to 39 of the 54 passengers and lone driver suffered mild to serious injuries.

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The trip to Carthage, Missouri, had been scheduled by the Vietnamese Catholic Martyrs Church in Houston, though some of the passengers may have belonged to other churches, she said.

The 52-year-old driver had his commercial driver's license, but his medical certificate had expired, she said.

Late Friday, police were assisting NTSB investigators with gathering evidence and mapping the scene, documenting the location of witness marks and scars on the highway and bridge rail over an overpass, she said.

Preliminary evidence suggests that the bus hit a rail on the right side of the bridge and then traveled 1,809 feet before coming to a stop on its right side in the earthen median between the highway and a frontage road, Hersman said.

"As it dropped off the bridge rail, the bus rolled to the right -- or the passengers' side -- where it came to rest."

The NTSB team includes a leader and six additional investigators, she said. They are gathering records about the vehicle, its maintenance and when and where it was serviced, she said.

She noted that the bus was not equipped with seat belts, which are not required on motor coaches.

Twelve people were declared dead at the scene of the wreck, another died at Parkland Hospital in Dallas and two more died at John Peter Smith Hospital in Fort Worth, officials said.

The bus was operating illegally, according to the Federal Motor Carrier Safety Administration. In a written statement, agency Administrator John H. Hill said investigators noted "certain irregularities ... about whether the bus was operating under the name Angel Tours or Iguala Busmex. It is important to note that neither of these domestic entities is authorized to operate as a U.S. passenger carrier in interstate commerce at this time."

He added that the agency identified Angel Tours "as being a high-risk carrier due to safety violations detected during roadside safety inspections and was subjected to an FMCSA compliance review in May 2008. This review resulted in FMCSA placing Angel Tours' operations out-of-service. To date, Angel Tours has not provided the agency with evidence of satisfactory corrective actions to the problems discovered and remains out-of-service."

In addition, "FMCSA has not granted Iguala Busmex authority to transport passengers because it has failed to fully comply with federal safety requirements."

He said police have been asked to stop any of the companies' buses.

Both companies are owned by Angel de la Torre.

A man who answered the phone at Angel Tours office said the owner was meeting with his lawyers and was unavailable.

The bus was on an overpass headed north on U.S. 75 when it left the highway about 12:45 a.m., Fair said.

Passenger ages ranged from 12 to 70, said Dr. Al Cardenas, chief emergency physician at the hospital. The driver was among the survivors.

Forty-five-year-old Leha Nguyen was among the passengers. The massage therapist was traveling by herself when she departed Houston on the bus at 8:30 p.m. Thursday for what was to have been her fourth trip to Carthage, Missouri.

After sandwiches were passed around, the group said prayers, and Nguyen began to drift off to sleep in her window seat next to an older woman, four rows behind the driver, she said.

About 11:45 p.m., the bus passed Dallas, "and as soon as we passed Dallas we were on the two-way freeway and I feel the bus ride a little bit fast and I have a feeling, not a safe feeling," she said.

She added, "I feel a little bit shaky, but I just let myself at peace and then go to sleep."

Her sleep didn't last long. "As I opened my eyes, I see the chair falling off and I was sitting right below the TV and I felt that somebody was laying on my leg and then right next to me there was a lady, she got her arm really crushed up and on top of her there was another lady, she cannot move."

Though her head hurt, Nguyen -- who left Vietnam in 1975 as a refugee -- was in better shape than many of her fellow riders. She accepted an ambulance ride to the hospital, where she learned that her seatmate had died. Once her scalp was sewn shut, she aided in translating for other Vietnamese patients whose English was not as good.